1. Field of the Invention
This invention relates to controlled release fertilizers and processes for their preparation. More particularly, it relates to particulate fertilizer compositions formed from nutrient granules (such as NPK granules) having intermediate layers or precoats and encapsulating or outer coats applied thereon. It also relates to processes for producing particulate fertilizer compositions that have desirable controlled release characteristics.
2. Description of Related Art
Fertilizers have been used for thousands of years to supplement nutrients in growing media. It has been known for some time that the benefit provided by the additional nutrient may depend on when it is delivered to the growing media and, in turn, made available to plants growing in the growing media. Sudden delivery of too much fertilizer can be wasteful or even detrimental to the plants. Delivery of too little fertilizer or delayed delivery of an adequate amount, on the other hand, may starve plants. It is desirable to provide particulate fertilizer compositions that deliver a relatively uniform rate of nutrient to the growing media over time or another specific release pattern.
Delivery of the correct amount of nutrient during an extended growing period previously required multiple applications of a relatively small amount of fertilizer compositions, a very labor intensive method. Accordingly, controlled release fertilizers were developed. Currently, it is desirable to apply the fertilizer once every few weeks to several months.
Generally, controlled release fertilizers used coatings around the nutrient granule to act as a physical or chemical barrier between the nutrient core and the ambient growing media The barrier delayed contact of the nutrient core with moisture and thereby delayed the moisture dissolving the core, and release of the nutrient into the growing media The rate of release of nutrient depended on the material used as the barrier, and its thickness and integrity, among other factors.
One approach to utilizing the barrier or encapsulating technique is shown in U.S. Pat. No. 3,223,518, issued Dec. 14, 1965 to Louis I. Hansen and assigned on its face to Archer-Daniels-Midland. The Hansen patent contemplates fertilizer products having single or multiple layers of a primer coating applied onto a nutrient core with single or multiple encapsulating layers. As described in the Hansen patent, the primer coating was fully cured before application of the encapsulating coating. The resulting fertilizer showed delayed release of the core nutrient as compared to a completely uncoated nutrient granule. However, repeated application of coatings and curing thereof has been found to be time consuming and commercially unfeasible. Furthermore, fertilizers made according to the teachings of Hansen, having only one primer and only one encapsulating coating, are disclosed by Hansen as releasing from 30-40% of the nutrient within 6 hours of contact with moisture and 50-60% within 24 hours. Such a product delivers too much nutrient too quickly to be acceptable for some controlled release fertilizer applications.
Another, later approach to attempting to provide an encapsulating layer with desirable release characteristics is described in U.S. Pat. No. 4,657,576, issued on Apr. 14, 1987, to Johannes M. H. Lambie and assigned on its face to Sierra Chemical Company. The core is encapsulated with a water-insoluble dicyclopentadiene based resin, such as that sold in the market by The Scotts Company in association with the trademark OSMOCOTE.RTM.. Although the use of a polymeric barrier is similar to the encapsulating coating described in the Hansen '518 reference, ingredients that regulate the pH of the coating were added. According to this reference, this improved some aspects of the release pattern.
A recognized shortcoming of the application of a polymeric barrier, such as the OSMOCOTE.RTM. encapsulating layer, to a nutrient core was that the release properties of the fertilizer was dependent on the quality of the core or substrate over which the polymer was applied. Discontinuities in the surface of the substrates, such as holes in prilled substrates, deformed granules, or particles having cracks, crevices or irregularities, created incomplete or non-uniform coverage by the coating. Typically for lower coating weights, such as 5 parts per hundred of the core weight ("PPH"), not enough encapsulating material is provided to adequately cover the defects in the core surface of low quality core granules and an unacceptable amount of imperfectly coated particles are created. Predictably, too many of the resulting particles in a fertilizer composition release too much nutrient in the first few days, making them inappropriate for some controlled release (delayed release) products.
Attempts to compensate for incomplete or non-uniform coverage of inexpensive low quality, irregular core granules by applying a thicker layer of the encapsulating coating was found to produce less than desirable results. Doubling the coating weight to 10 parts per hundred, for example, will more efficiently cover the surface defects on more of the granules. However, this thicker outer coating so efficiently seals the nutrient in such a large number of the granules as to either cause "lock off" or to prevent release of nutrient from the core into the growing media during an overly long period of time to be commercially acceptable.